SPACE PHYSIOLOGYWe have already described in the previous chapter how space flight affects the organ systems of the body (the heart and the lungs), and we even discussed how some of the smaller structures of the body, the blood vessels, are affected in space. Now we are taking our discussion to even smaller, microscopic levels by examining flight affects our body at the cellular level. We know that space flight somehow affects the activity of the cells in our blood, including the RBCs. Let's examine how the erythrokinetics of space flight is different from that on Earth.What has surprised scientists is the fact that, for space travelers, the percentage or concentration of RBCs in the bloodstream stays about the same even though plasma volume decreases in space. That is, the "space normal" hematocrit is about the same as the "Earth normal" hematocrit. What does this suggest? Remember that as humans are exposed to the microgravity of space, there is a loss of body fluid (including a loss of plasma) due to the headword fluid shift One would imagine that as the plasma level is decreased in the bloodstream, then the concentration of blood cells in the bloodstream would increase. If this were true, the hematocrit would rise. Data has indicated that the inflight hematocrit measurements for astronauts do not change to any appreciable extent from the preflight hematocrit, which means that the percentage of RBCs in the blood inflight is not different from the percentage of RBCs in the blood preflight, even though there is less plasma. Since there is a decrease in plasma volume, and since the hematocrit does not change, this suggests that the number of RBCs must decrease. We call this reduction in RBCs "space anemia." There are different theories that exist to explain this decrease in the number of RBCs. One, called the "hemoconcentration" theory, suggests that, while in space, the body detects an overabundance of fluids in the upper part of the body. The astronauts find that they are not thirsty and they want to drink less while, at the same time, their kidneys are stimulated to remove this excess fluid, part of which is plasma. We have already learned a great deal about this in the previous chapters. The removal of plasma causes the blood to becomes "thicker," because as fluid is eliminated, the percentage of RBCs per volume of blood increases. This may cause an overabundance of oxygen- carrying ability. When the kidney detects this overabundance of oxygen, the kidney reduces the production of erythropoietin, which, in turn, suppresses RBC formation. This theory suggests that the production level of RBCs decrease. Data suggests that this is not the only theory to consider. There have been alternative suggestions made to explain the space flight reduction of RBCs. These include the possibility that the "space anemia" is due to the loss of muscle mass which occurs in space flight. Because muscles are used less in microgravity (there isn't even any "walking around" to do in space) the muscles lose mass and require less oxygen. With a lower oxygen requirement, the blood can reduce its oxygen-carrying capacity. This theory suggests that the body responds to this lower oxygen requirement by reducing the number of RBCs produced. If the last theory we discussed is true, that the muscles require less oxygen, another possible explanation of how the blood can reduce its oxygen-carrying capacity would be to increase the destruction rate of RBCs. If this theory is correct, then the proportion of circulating reticulocytes (immature RBCs) in the bloodstream would increase above normal. This is because the normal production line in the marrow would continue kicking out reticulocytes, while at the same time, many of the mature, healthy RBCs already in the bloodstream would be destroyed. A fourth theory for the reduction in RBC mass has to do with the well-documented fact that astronauts lose calcium from their bones under conditions of microgravity. (We'll be talking more about this in a later chapter.) The loss of body calcium could disrupt the bone architecture, which would result in the loss of bone strength, a condition that is similar to a disease called osteoporosis. This alteration of bone metabolism may also affect the bone marrow, and, therefore, affect normal RBC production in the marrow. To summarize the theories, a decrease in the number of RBCs can occur in space under the following conditions:
(b) as muscle mass is lost and their oxygen requirement is reduced, the kidneys detect an overabundance of oxygen-carrying capacity in the blood, which may cause them to suppress the production of erythropoietin, resulting in a decrease in RBC production; (c) for the same reason shown in (b), the body may respond to the overabundance of oxygen-carrying capacity in the blood by increasing the destruction rate of RBCs; and (d) as astronauts lose calcium in their bones, the structure and function of the bone and its marrow may change and may result in a decrease in RBC production.
A detailed investigation developed by Dr. Clarence Alfrey and his research team was designed to examine the effect of microgravity and the interaction of changes in body weight and plasma volume, which both decrease inflight, on the rate of RBC production. His experiment was also designed to examine the roles of the hormone erythropoietin in the production or reduction of circulating RBCs. The investigation measured:
Hypothesis 1
Hypothesis 2
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